1. Field of the Invention
The present invention relates to electrode structure of a plasma display panel, referred to hereinafter as a PDP, and its driving method.
2. Description of the Related Arts
PDP is a thin display device of self-luminescent type which allows a high-speed display so as to suit the television.
AC type color A PDPs of a surface discharge type have been popularly employed, resulting in a rapid increase in application od PDP's. Accordingly, there has been required further improvement of the picture quality, such as 256 gradations, so as to achieve large screens of High-Definition TV and computer display, etc.
An electrode matrix of an AC type surface-discharge PDP is composed of plural pairs of first and second sustain electrodes X & Y extending in a first direction of display lines and address electrodes A extending in a second direction of rows orthogonal display lines, respectively, as shown in FIG. 1.
The sustain electrode pairs are provided on a first substrate of a substrate pair which composes the panel envelope. A space S1 between the first and second sustain electrodes Xn & Yn, referred to hereinafter as a discharge slit, constitutes respective lines. The suffix n indicates the electrodes are of the n-th line. However, the suffix n may be hereinafter omitted unless the line must be specifically distinguished from the adjacent line, for the explanation.
Both the sustain electrodes are covered with a dielectric layer extending along the entire screen so as to be insulated from a discharge space formed between the first and second substrates.
A single cell C is formed at the intersection of each discharge slit and each address electrode, including the vicinity of the intersection. The cell C is a single unit luminous area.
A memory effect of each cell is utilized to maintain, i.e. to sustain, the lighting state of the cell.
AC type PDPs are constituted so as to possess structurally the memory function by covering the display electrodes with the dielectric layer.
In operating the AG type PDPs, after the wall charges are accumulated only on the above-mentioned dielectric layer of the cell which should light according to the data of the display, voltages of alternating polarity, i.e, sustain voltages, are applied commonly across the cells. This sustain discharge is the surface discharge along the surface of the above-mentioned dielectric layer.
The sustain voltage is lower than the firing voltage between the sustain electrodes. In the cells having the wall charges, a voltage generated by the wall charge is superposed onto the sustain voltage, therefore, an effective voltage, referred to hereinafter a cell voltage, across the cell exceeds the firing voltage so that a discharge is caused therein.
The wall charges having the polarity opposite from the previous state accumulate, after the previous wall charges disappear once in the discharge. Therefore, each time the sustain voltage is alternately applied, the discharge takes place. When the application cycle of the sustain voltages is shortened, a visually continuous lighting state is achieved.
FIG. 2 schematically illustrates an internal structure of a typical prior art PDP of the surface discharge type. In PDP 90, a plurality of pairs of first and second sustain electrodes 93 & 94 are arranged on the inner surface of a front glass substrate 91 so as to extend along the direction of the lines of the display matrix, that is in the direction vertical (i.e., perpendicular) to the sheet of FIG. 2.
A space between the electrode pair, that is a discharge slit, forms a single line. These matters will be described later in detail.
There is provided a dielectric layer 96 so as to insulate sustain electrodes 93 & 94 from discharge space 99. A protection film 97 is deposited on the surface of dielectric layer 96. Dielectric layer 96 and protection film 97 are both transparent.
On the other hand, in the inner surface of back glass substrate 92, address electrodes 95 are arranged orthogonal to sustain electrodes 93 & 94. A fluorescent layer 98 is provided so as to cover back glass substrate 92 and the surface of address electrodes 95. The fluorescent layer thus located far from the surface discharge can decrease the deterioration of the fluorescent layer caused from ion bombardment thereto.
The address electrodes are arranged generally on the substrate of the side where the fluorescent layer is coated, in order to avoid an increase in the electric power consumption caused from stray capacitance between the sustain electrodes and the address electrodes.
The first sustain electrode 93 is composed of a belt of a metal film 932 narrower than a belt of a transparent conductive film 931. The second sustain electrode 94 is similarly composed of a belt of a metal film 942 narrower than the belt of a transparent conductive film 941. Metal films 932 & 942 are supplementary conductors to accomplish good electrical conductivity, and are stacked on an edge of a side apart from the surface discharge slit between transparent conductive films 931 & 941, respectively.
In a display operation of PDP 90, addressing operation is usually performed sequentially in the order of the lines. In lighting a certain cell, the address electrodes 95 and the second sustain electrodes 94 associated with respective cells are appropriately biased so as to cause an opposing discharge in the direction of the thickness of the panels so that the wall charges are accumulated on the surface of dielectric layer 96, where protection film 97 is assumed to be a part of dielectric layer 98.
When the cell is not to be lit, the voltage of each electrode is set so as to not cause the opposing discharge with address electrodes 95. After the address period in which lighting/non-lighting of the cell is thus set, the sustain voltages are applied between the first and the second sustain electrodes 94 and 93 so that the polarity of the voltages applied between these sustain electrodes may change alternately, whereby the surface discharge takes place along the dielectric layer ever the discharge slit upon each transition of the applied voltages.
Fluorescent layer 98 is partially excited by ultraviolet rays UV generated by the discharge so as to radiate a visible light of a predetermined color. Among these visible lights the light that has penetrated through front glass substrate 91 becomes a display light.
Lighting efficiency can be improved by expanding the surface discharge area while suppressing the shading of the display light to the minimum, by constituting first and the second sustaining electrode 93 & 94 located on the front side of discharge space 99 with the above-described layered structure.
A part, in the line direction, or discharge slit S1 is a surface discharge gap. The width or discharge slit S1, that is a size in a direction perpendicular from sustain electrode 93 & 94, is selected so that a surface discharge may be properly caused by an application of a driving voltage of 100 to 200 V.
On the other hand, a slit S2 between an n-th second sustain electrode 93 and the nest n+1 th first sustain electrode 94 of the adjoining line is called "a reverse slit"; and the width of reverse-slit 62 is selected so wider enough than that of discharge slit 81 that no discharge is generated across reverse-slit S2. Each line can be selectively discharged by thus providing discharge slit S1 and reverse-slit S2 in the arrangement of the first and second sustaining electrodes 93 & 94.
The opposing discharge, referred to herein after 95 an address discharge, in the addressing operation is initiated between address electrode 95 and the second metal film 942 of second sustain electrode 94, and then, as the wall charges accumulate upon the surface or insulating layers 96 and 97, shifts to a discharge between address electrode 95 and second transparent conductive film 941.
The address discharge terminates when the electric field in discharge space 99 becomes weak due to the accumulation of the wall charges above the second transparent conductive film 941, in the direction to cancel the applied electric field.
The reason why the discharge takes place first between the second metal film 942 and address electrode 95 is that second metal film 942 is located nearer to address electrode 95 than the second transparent conductive film 941. Charging current flows into second sustain electrode 94 before starting the address discharge because discharge space 99 is a kind of condenser.
Second metal film 942 is of lower resistance than that of second transparent conductive film 941; accordingly, the current density of second metal film 942 is larger than that of the second transparent conductive film 941. Therefore, in the vicinity of the second metal film 942, there is generated a stronger electric field than in the vicinity of second transparent conductive film 941, whereby the discharge takes place easily.
However, as the quantity of the lines increases, to meet demands for higher resolution of the screen, the length of the period, allocatable in addressing a single line within the display period or a single frame, decreases the wall charges accumulated in the vicinity of slit S1 during the discharge are and, accordingly, lighting errors are more likely to take place, resulting in emitting no light during subsequent sustain period.
This is because, when the addressing period is short, the voltage application to the electrode is released thereby to terminate the address discharge before the address discharge shifts to the discharge between transparent conductive film 941 and address electrode 95 where the charge must be saturated.
The increase in the number of gray scale grades also causes the shortening of the addressing period.
Moreover, in the prior art structure there was another problem in that erroneous lightings of surface discharge cells of adjacent lines are likely to take place because comparatively much of the wall charges accumulate upon the insulating layer on reverse-slit S2, too.
In addition, in the prior art there was a further problem in that the applied voltage, necessary to cause the address discharge between the address electrode and the sustaining electrode, has to be large.
Therefore, it was difficult to increase the space size of the discharge slit S1 in order to improve the brightness of the cell.